Studies with rat hepatoma cell cultures indicate that the nonsteroid anti-inflammatory drugs, in therapeutic and toxic concentrations, reversibly inhibit cell proliferation. A parallel reduction in protein and nucleic acid synthesis is also observed with the exception that, in low concentrations, the salicylate drugs salicylamide, salicylic acid and aspirin stimulate protein and nucleic acid synthesis. In high concentrations, the salicylate drugs inhibit protein and nucleic acid synthesis as well as cell proliferation. The order of potency in inhibiting culture growth, meclofenamate greater than indomethacin greater than salicylamide greater than phenylbutazone greater than phenacetin greater than aspirin (salicylic acid), is similar to that reported for their anti-inflammatory activity and their ability to inhibit prostaglandin synthesis. The findings, therefore, may have some bearing on the anti-inflammatory activity of these drugs.